Hydroxylated rubbers, for example, hydroxylated polybutadiene are known to improve the properties of silica filled mixtures. For example, in GB 1,575,115 there is disclosed a process for the production of a hydroxy-modified polybutadiene involving reacting a polybutadiene having a vinyl double bond content of from 30 to 60 percent with a hydroxy-mercaptan compound. Examples of suitable mercaptans disclosed in this reference include 2-mercaptoethanol, 2-mercapto-2-propanol, 1-phenyl-2-mercaptoethanol, 3-mercapto-propanol and 4-mercaptobutanol. The process is carried out in an inert solvent, 2.3-9.0 parts by weight per part of polybutadiene, and is initiated by a free radical catalyst such as azobisisobutyronitrile (AIBN). The AIBN catalyzed reaction of 2-mercaptoethanol with polybutadiene in an inert solvent has, in fact, been known for many years (C.S. Marvel, K.G. Clarke, H.K. Inskip, W.K. Taft and B.G. Labbe, Industrial and Engineering Chemistry, Volume 45, No. 9, pp. 2090-2093 (1953)).
In these prior art processes, the use of relatively large amounts of solvent has practical disadvantages, including the need for large volume reactors, as well as auxiliary equipment for the separation and purification of the solvent. Solvent recovery generally requires a distillation step, contributing substantially to energy costs. Furthermore, relatively long reaction times, 2 hours or more, are employed in these solvent-based processes thus limiting production efficiency. Therefore, there exists a need for a process for the efficient hydroxylation of vinyl-containing rubbers which avoids the disadvantages of the prior art.